Modal logic is actually quite useful. If modal realism turns you off you can just accept it as a language game (which any sort of formal logic is going to be.) The non-sequitur in Plantinga's argument, as presented by cousin it, is P3. (Plantinga's own argument is a bit more subtle, and its ultimate error is in eliding between different meanings of the term "possible." He successfully shows that under formal logic if possibly necessarily x then necessarily x, and then ascribes possible necessity to God because God is one of the most few things that often is argued to be necessary, and that God seems like the sort of sufficiently abstract thing that it might be necessary. But this isn't the sort of possibility that's germane to formal logic.)